Recent Studies Show Global Warming May Enhance Soil Carbon Storage and Thereby Slow Its Own Progression
The amount of carbon stored above and beneath a unit area of land is basically a function of two biochemical processes, photosynthesis and respiration. During photosynthesis, plants remove CO2 from the atmosphere and utilize it to construct their tissues, where it is safely retained until it is respired back to the atmosphere. Thus, if the total amount of photosynthesis occurring over a given area of land is greater than the total amount of respiration occurring above and beneath its surface, that area of land is said to be a carbon sink. Conversely, if the amount of photosynthesis is less than the amount of respiration, the area is said to be a carbon source.

For many years, theoretical models of ecosystem dynamics suggested that global warming would reduce both the magnitude and number of terrestrial carbon sinks by increasing ecosystem respiration more than it increased ecosystem photosynthesis. If true, this result would dash all hopes of mitigating CO2-induced global warming via biological carbon sequestration. However, like model-based predictions of climate change, there are a number of problems with this prediction as well.

The primary problem is the simple fact that most observational evidence does not support the model predictions of reduced soil carbon storage under elevated temperatures. Fitter et al. (1999), for example, evaluated the effect of temperature on plant decomposition and soil carbon storage, finding that upland grass ecosystem soils artificially heated by nearly 3°C increased both root production and root death by equivalent amounts. Hence, they concluded that in these ecosystems, elevated temperatures "will have no direct effect on the soil carbon store." Similarly, Johnson et al. (2000) warmed Arctic tundra ecosystems by nearly 6°C for eight full years and still found no significant effect of that major temperature increase on ecosystem respiration. Furthermore, Liski et al. (1999) showed that carbon storage in soils of both high- and low-productivity boreal forests in Finland actually increased with warmer temperatures along a natural temperature gradient.

Why the big discrepancy between model predictions and reality? According to a recent paper in the Annals of Botany, there are two potential explanations: (1) ecosystem modelers are over-estimating the temperature dependency of soil respiration, and (2) warming may increase the rate of certain physico-chemical processes that transfer organic carbon to more stable soil organic matter pools, thereby enabling the protected carbon to avoid or more strongly resist decomposition (Thornley and Cannell, 2001).

That the first of these explanations is viable is demonstrated by the results of the studies just described. The second explanation is also reasonable. Thornley and Cannell hypothesize, for example, that the pertinent physico-chemical processes require a certain amount of activation energy to attach organic materials onto soil minerals or bring them together into aggregates that are less subject to decomposition; and they suggest that higher temperatures can provide that energy.

Taking their hypothesis one step further, Thornley and Cannell developed a dynamic soil model in which they demonstrate that if their thinking is correct, "long-term soil carbon storage will appear to be insensitive to a rise in temperature, even if the respiration rates of all [soil carbon] pools respond to temperature as assumed by [most models]," which is, in fact, what experimental and real-world data clearly indicate to be the case.

The upshot of these several observations is that global warming does not cause terrestrial carbon sinks to release additional CO2 to the atmosphere and thereby exacerbate the warming, as was fervently believed up until the last few years. In fact, it is much more likely that rising temperatures may do just the opposite, inducing a negative feedback phenomenon that enables greater amounts of carbon to be sequestered, which would tend to decrease the rate of CO2-induced warming.

Clearly, the biosphere is well adapted to responding to environmental challenges; and this one is no exception. When the going gets hot, the earth knows how to keep its cool.